A241804 Number of ballot sequences of length 2n having exactly n descents.

1, 0, 0, 1, 9, 112, 1582, 26159, 486170, 10131071, 233582056, 5895589738, 161776346286, 4794754890798, 152576687930997, 5187562649814737, 187666786444911220, 7196725552061993554, 291572346138097726746, 12443163766976461627878, 557880276243757014820691

Offset: 0

Author

Joerg Arndt and Alois P. Heinz, Apr 28 2014

Keywords

• nonn

Examples

			a(3) = 1: [1,2,1,3,2,1].
a(4) = 9: [1,2,1,2,1,3,2,1], [1,2,1,3,1,4,2,1], [1,2,1,3,2,1,2,1], [1,2,1,3,2,1,3,1], [1,2,1,3,2,1,3,2], [1,2,1,3,2,1,4,1], [1,2,1,3,2,1,4,2], [1,2,1,3,2,1,4,3], [1,2,1,3,2,4,3,1].
		

Formula

a(n) = A238121(2n,n) = A238122(2n,n).